Thursday, March 14, 2013

Dual-Shock Quark Nova

A super-luminous supernova (SLSN) is a class of supernova whose peak luminosity is several times larger than a typical supernova. Mechanisms that can give rise to a SLSN include the explosion of a very massive star in a pair-instability supernova, the interaction of supernova ejecta with circumstellar matter or a dual-shock quark nova (dsQN) event. SN 2006oz is currently the only SLSN known to exhibit a double-humped lightcurve that is consistant with a dsQN model. The lightcurve of SN 2006oz can be explained by a quark nova occurring 6.5 days after a core-collapse supernova explosion. A dsQN event like SN 2006oz is very rare since it is estimated to occur at a rate of 1 in every 10,000 core-collapse supernovae.

In a dsQN model, a massive star explodes
in normal core-collapse supernova and leaves behind a rapidly-spinning,
high-mass neutron star. As the neutron star spins down, its central density
gradually increases. This eventually leads to a detonative phase transition
known as a quark nova, where the neutron star violently converts into a quark
star. During this process, the neutron star’s outer layer is ejected at
ultra-relativistic velocities. An enormous amount of kinetic energy is carried
away since the quark nova ejecta consists of ~100 Earth masses of
material travelling close to the speed of light.

Although the quark nova occurs several
days after the core-collapse supernova, ejecta from the quark nova travel many
times faster than the supernova ejecta. The quark nova ejecta rapidly catch up
and collide with ejecta from the preceding supernova. This re-shocks the
supernova ejecta and leads to a rise in luminosity over an extended period of
time. As a result, a SLSN consisting of a normal core-collapse supernova
followed by a quark nova is characterised by a double-humped lightcurve. The
fainter first hump corresponds to the core-collapse supernova while the
brighter second hump corresponds to the re-shocked supernova ejecta.

A double-humped lightcurve indicative of
a dsQN is only produced when the quark nova happens ~10 days after the
core-collapse supernova. If the time interval between the supernova and quark
nova is too long, the supernova ejecta would have dissipated so much that the
quark nova basically occurs in isolation. In contrast, if the time interval is
too short, the two lightcurves would overlap and prevent a distinct double-hump.